With the development in the digital technology in recent years, electronic devices such as mobile information-processing equipment and home information appliances have increasingly been sophisticated. With the sophistication of these electronic devices, there has been a rapid progress in the size, integration, and operation speed of the nonvolatile storage devices used for these electronic devices. In addition, the application of the nonvolatile storage devices has rapidly been expanded.
Among them, a storage device has been proposed which use, as a storage element, non-volatile variable resistance elements provided in a matrix therein, and there has been expectation for further progress in the size, integration, and operation speed as three-dimensional memories.
The variable resistance element includes a thin film composed of a material mainly made up of a metal oxide. The electrical resistance value of the thin film changes upon application of electric pulses, and the changed electrical resistance value is stored. Thus, when a high resistance state and a low resistance state of the thin film correspond to binary data “1” and “2”, respectively, the binary data can be stored in the variable resistance element. It is to be noted that the current density of electric pulses applied to the thin film of the variable resistance element and the magnitude of the electric field generated in response to the application of electric pulses are only necessary to be a degree sufficient to cause a physical state of the thin film to change and not to damage the thin film.
In addition, the variable resistance element that takes binary values includes (i) a variable resistance element that changes its resistance value in response to application of electric pulses having the same polarity and different voltages (so-called unipolar type) and (ii) a variable resistance element that changes its resistance value in response to application of electric pulses having different polarities (so-called bipolar type). In general, the unipolar type of variable resistance element has characteristics of which writing when changing from the low resistance state to the high resistance state (so-called reset) takes longer amount of time than writing when changing from the high resistance state to the low resistance state (so-called set). On the other hand, writing in the bipolar type of variable resistance element can be carried out in short amount of time in both of the set and reset.
In a storage device including such a variable resistance element (so-called a cross point storage device) provided at a corresponding one of three-dimensional crosspoints of word lines and bit lines which orthogonally cross without coming in contact with each other, there is, in some cases, a disorder that, when data is written into a variable resistance element, the electrical resistance value of another variable resistance element changes due to a bypass current. Hereafter, the disorder is called “write disturb”. Thus, when such a cross-point storage device is configured, it is necessary to separately provide a particular configuration in order to prevent the write disturb from occurring.
Since the resistance of the unipolar type of variable resistance element can be changed by applying electric pulses having the same polarity, it is possible to prevent occurrence of the write disturb by arranging unipolar current steering elements, such as a PN junction diode or a Schottky diode, in series to the variable resistance element. The unipolar current steering elements have nonlinear voltage-current characteristics including the high resistance state and the low resistance state in a voltage range of one polarity of a voltage.
A storage device is disclosed which includes a storage element composed of a series circuit of a variable resistance element and a Schottky diode (a current steering element), as a storage device capable of preventing occurrence of such write disturb (see PTL 1, for example).
In the proposed storage device, the bypass current flowing into the variable resistance element of storage elements other than the storage element into which data is to be written (selected storage element) is blocked by the Schottky diode. This prevents occurrence of write disturb in the cross point storage device. Here, in the proposed storage device, writing of data into the variable, resistance element is carried out by applying, to the variable resistance element, electric pulses having the same polarity. Thus, writing of data is not disturbed by the Schottky diode connected in series to the variable resistance element.
On the other hand, since bipolar electric pulses are used for writing into the variable resistance element when the bipolar type of variable resistance element is used, a bipolar current steering element needs to be provided in series to the variable resistance element. The bipolar current steering element has nonlinear voltage-current characteristics including the high resistance state and the low resistance state in a voltage range of a positive polarity and a negative polarity. A two-terminal device such as metal-insulator-metal (MIM) diode, metal-semiconductor-metal (MSM) diode, and a varistor is known as an element having such characteristics.
FIG. 25A and FIG. 25B is a characteristic diagram schematically showing voltage-current characteristics of the current steering to element. FIG. 25A is a voltage-current characteristic diagram of a bipolar current steering element such as the MIM, the MSM, or the varistor, and FIG. 25B is a voltage-current characteristic diagram of a Schottky diode.
As shown in FIG. 25B, the Schottky diode shows nonlinear electric resistance characteristics, however, the voltage-current characteristics are not symmetric at all with respect to the polarity of applied voltages.
On the other hand, the two-terminal device such as the MIM diode, the MSM diode, and the varistor shows the nonlinear electric resistance characteristics and the voltage-current characteristics substantially symmetric with respect to the polarity of applied voltages. That means that each of change in a current with respect to a positive applied voltage and change in a current with respect to a negative applied voltage becomes substantially symmetric with respect to an original point. In addition, in those two-terminal devices, the electric resistance is significantly high in a range (the range C) in which an applied voltage is equal to or lower than a first critical voltage (the lower limit voltage in the range A) and equal to or higher than a second critical voltage (the upper limit voltage in the range B), whereas the electric resistance rapidly decreases when exceeding the first critical voltage or falling below the second critical voltage. More specifically, these two-terminal devices have nonlinear electric resistance characteristics with which large current flows when an applied voltage exceeds the first critical voltage or falls below the second critical voltage.
Thus, using these two-terminal devices as bipolar current steering elements makes it possible to avoid occurrence of write disturb in a cross point nonvolatile storage device that uses a bipolar variable resistance element capable of a high-speed operation in both setting and resetting.
In a storage device of a variable-resistance type, comparatively large current is usually required to flow through the variable resistance element when writing data into a variable resistance element, although it highly depends on a material, a configuration, and so on of the variable resistance element, in order to change the electric resistance value by applying electric pulses to the variable resistance element to change the state of the variable resistance element from a high resistance state to a low resistance state. It is disclosed, for example, that, in an operation of a storage device including a variable resistance element, current is caused to flow, using a varistor, with a current density equal to or higher than 30000 A/cm2 when writing data into the variable resistance element (see PTL 2, for example). In recent years, various considerations have been made to reduce the current necessary for writing data into a variable resistance element. As a result, it is now considered that the current density equal to or higher than 30000 A/cm2 is not necessarily required as a current density of current necessary for writing data into the variable resistance element. However, significantly large current of 10000 to several tens of thousands of A/cm2 is generally required when writing data into the variable resistance element.